The present invention relates to a method of mass spectrometry and a mass spectrometer.
It is known initially to calibrate a mass spectrometer. A known initial calibration routine involves utilising a calibration file in conjunction with a number of known compounds. Different known species of ions having different mass to charge ratios are mass analysed and the time of flight or mass to charge ratio of the different species of ions is determined. The correspondence between the measured time of flight or the mass to charge ratio of the known different species of ions and the theoretical mass to charge ratio of the ions as held in the calibration file is determined. A calibration curve is then fitted and adjusted to minimise the errors between the experimentally determined values and the theoretical values of the initial calibration compounds. In particular, a 5th order polynomial calibration curve may be fitted to the experimental data and the terms of the 5th order polynomial calibration curve may be adjusted so that the RMS error is as low as possible. The calibration curve is then used in subsequent mass analyses.
During subsequent operation of a mass spectrometer the mass spectrometer may experience changing conditions which can potentially have a significant impact upon the measured time of flight (and hence determined mass to charge ratio) of ions by the Time of Flight mass analyser. In particular, a temperature change of 1° C. can shift the measured time of flight and measured mass to charge ratio of all ions by approximately 40 ppm.
In order to address this problem it is known during subsequent operation of a mass spectrometer to periodically check the determined time of flight or mass to charge ratio of a known lockmass ion. If the mass spectrometer determines that the measured time of flight or mass to charge ratio of the known lockmass ions has shifted, then the measured time of flight or mass to charge ratio of all ions is then globally adjusted to correct for the shift. The adjustment which is applied is a global adjustment to the measured mass to charge ratios of all ions and reflects the fact that there has been a global shift in measured mass to charge ratios due e.g. to an increase in temperature.
The known calibration approach and subsequent lockmass correction method is imperfect and different residual calibration errors will remain at different mass to charge ratios.
FIG. 1 shows some of the residual calibration errors following an initial conventional calibration routine. It is apparent that the residual calibration errors may typically be a few ppm.
One problem with the known lockmass correction approach is that it can introduce systematic errors.
Conventional mass spectrometers which seek to correct for global shifts by using lock components adjust the mass spectral data to correct for any discrepancy between the measured mass to charge ratio of the lockmass ions and the theoretical mass to charge ratio of the lockmass ions. However, this approach to lockmass correction can inadvertently result in systematic errors being introduced through a variety of sources particularly mass calibration residuals.
GB-2494492 (Micromass) discloses a method to single point internal lock-mobility correction.
GB-2406966 (Klee) discloses a method of correcting spectral skew in a mass spectrometer.
U.S. Pat. No. 6,519,542 (Giannuzzi) discloses a method of testing an unknown sample with an analytical tool.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.